The manufacture of thin film transistor-liquid crystal displays (TFT-LCDs) has continued to attempt to replace amorphous silicon used as a TFT channel material with polysilicon to achieve higher performance, larger scale integration and faster response. However, leakage currents tend to occur at polysilicon grain boundaries and it becomes necessary to produce large-sized polysilicon grains to reduce grain boundaries. Conventional laser-based melting and crystallization can produce a mono-crystalline silicon film with a length of up to several micrometers, but cannot selectively produce mono-crystalline silicon grains at desired locations. To increase a grain size, a sequential lateral solidification (SLS) process has recently been suggested.
According to a suggested SLS process, a predetermined region of an amorphous silicon film is treated with a laser beam using a mask for crystallization and the mask is then moved at a pitch smaller than that of the crystal formed. Then, the amorphous silicon film is again treated with a laser beam using the formed crystal as a seed. In this way, the crystal size continuously grows. The silicon grains formed by the first shot of laser beam act as seeds, allowing lateral re-growth of mono-crystalline silicon in the amorphous silicon film. Theoretically, the SLS process can yield very long mono-crystalline silicon.
In a conventional SLS process for crystallization of amorphous silicon, a mask composed of a quartz plate and a chromium film pattern coated on the quartz plate is used. To crystallize amorphous silicon, a laser beam having high energy density is used. However, the crystallization of amorphous silicon using a laser beam with high energy density greatly reduces the durability of the mask.